1. Field of the Invention
The present invention relates to fiber optical communication systems, and, in particular, to the selection of optical sources for such systems.
2. Description of the Related Art
Fiber optical communication systems typically have optical sources, such as semiconductor lasers, that convert information-bearing electrical signals into modulated light for transmission along optical fibers to remote optical receivers that convert the light signals back into electrical signals for further processing (e.g., conversion into sound at a telephone). One or more optical amplifiers may be placed along a given transmission path between an optical source and a corresponding optical receiver. These optical amplifiers boost the light signals that become attenuated during transmission along an optical fiber. Optical amplifiers are used to increase the total distance between an optical source and an optical receiver.
Every element in a transmission path, including the optical source, the optical amplifiers, and the optical fibers themselves, introduces some degree of noise and/or pulse distortion into the light signals. When noise or distortion gets sufficiently large, bit errors will be introduced into the digital information carried by the modulated light signals. The frequency with which these bit errors occur (i.e., the bit error rate (BER)) is a useful measure of the quality of the transmission process. The higher the BER, the lower the quality. Although some of these bit errors may be corrected using auto-correction algorithms implemented at the optical receiver, at some point, the optical receiver will not be able to accurately recover the data with 100% accuracy and information will be lost.
One way to increase the total length of an optical transmission path without exceeding an unacceptable BER level (i.e., the BER floor) is to use a high-quality optical source. Higher quality optical sources introduce less noise into the optical path, thereby allowing more optical amplifiers to be placed in the optical path without reaching the BER floor, thereby enabling a longer transmission path.
Conventionally, chirp testing has been applied to characterize the quality of individual optical sources to determine their acceptability for different types of applications. The chirp of an optical source is defined as the variation in wavelength of a light signal generated by the optical source. The lower the variation, the lower the chirp. In the past, test measurements of the chirp of optical sources have been used to determine the acceptability of optical sources for applications with longer transmission paths and/or lower BER requirements, with optical sources having low chirp being selected for such applications.
Another way to characterize the quality of optical sources is to perform BER testing in which an entire optical transmission path from an optical source to an optical receiver is tested, including the optical fibers and any intervening optical amplifiers. Such testing of the entire optical transmission path greatly increases the cost and complexity of characterizing the quality of optical sources.
Currently, dense wavelength division multiplexed (DWDM) systems are becoming the preferred method for expanding the information capacity on existing single-mode fibers. For systems at 2.5 Gigabits per second (Gb/s), the number of channels has recently increased from 8 to 80, and the channel spacing has decreased from 200 Gigahertz (GHz) to 50 GHz, with plans to increase the transmission capacity by further decreasing the channel spacing. Commercial fiber systems are being planned to transmit data without regeneration on non-dispersion shifted fibers along transmission paths having a total length of up to 640 km. Such systems require optical sources, such as electro-absorption modulator isolated laser modules (EMILMs), with high quality and high reliability at an increasing number of wavelengths in the range of 1.55 micrometers (xcexcm). Conventional chirp testing alone has not proven to be an acceptable method for selecting EMILM devices for such applications. In particular, EMILM devices with relatively low chirp have been found to produce unacceptably high BER levels at transmission path lengths of 640 km.
The present invention is directed to a process for selecting optical sources, such as EMILM devices, for use in optical transmission systems, especially those with long transmission paths and/or low BER requirements. In particular, the present invention involves the application of specific noise testing to the optical sources, preferably in conjunction with conventional chirp testing, to characterize the acceptability of the optical sources for different applications. The specific noise testing involves characterizing the noise level in the light signals generated by an optical source for frequencies outside of bandwidth of the system in which the optical source will operate. For example, this noise testing may involve measuring the relative intensity noise (RIN) of the optical source at the relaxation oscillation frequency Fr. RIN is the average noise power divided by the average power of the optical source at a given frequency. Fr is the frequency that corresponds to the inherent peak noise for the optical source when operated in an unmodulated mode (e.g., continuous wave). The measured RIN may then be used to determine the acceptability of the optical device for different applications. The inventors have found that optical devices should have both low chirp and low RIN at Fr to be acceptable for fiber optical communication systems having long transmission paths, e.g., up to 640 km or even higher.
In one embodiment, the present invention is a method for selecting optical sources for use in a fiber optical communication system having an operating bandwidth, comprising the steps of (a) characterizing a plurality of optical sources with respect to a noise measure at a frequency outside of the operating bandwidth of the communication system to determine a noise measure level for each optical source; and (b) selecting one or more of the optical sources for use in the communication system based on the corresponding noise measure levels.
In an alternative embodiment, the present invention is a fiber optical communication system having an operating bandwidth, comprising one or more transmission paths, each transmission path comprising an optical source and an optical receiver with two or more optical fibers separated by one or more optical amplifiers between the optical source and the optical receiver, wherein, for each transmission path, the optical source has a noise measure level at a frequency outside of the operating bandwidth of the communication system that is below a specified threshold level.